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Subject: Re: [virtio] [PATCH v2] initialization: add extra device status handshake


On Wed, Sep 25, 2013 at 11:54:07AM +0930, Rusty Russell wrote:
> "Michael S. Tsirkin" <mst@redhat.com> writes:
> > In the past we sometimes wanted an ability to make driver
> > gracefully fail early in initialization if it does not
> > recognize/support a feature.  One such example we are introducing
> > right now is the new VERSION_1 feature.
> >
> > Another example where the driver configuration
> > is inconsistent with what the device can support.
> >
> > This modifies the initialization to allow
> > graceful failure in such cases:
> > 1. require a driver to first ack all features it recognizes
> > (linux does this already)
> 
> No it doesn't, but yes it should :)
> 
> > 2. require driver to set status after configuration
> > and re-check status afterwards
> 
> I like this, but I prefer the simpler approach:
> 
> 1) Call the new status bit FEATURES_OK.  This makes it fairly clear that
>    it's the end of feature negotiation.
> 
> 2) The driver SHOULD re-read the status to check FEATURES_OK has
>    "stuck".
> 
> How's this?
> 
> diff --git a/virtio-v1.0-wd01-part1-specification.txt b/virtio-v1.0-wd01-part1-specification.txt
> index b4298bb..83042c4 100644
> --- a/virtio-v1.0-wd01-part1-specification.txt
> +++ b/virtio-v1.0-wd01-part1-specification.txt
> @@ -106,6 +106,9 @@ This field is 0 upon reset, otherwise at least one bit should be set:
>    may be a significant (or infinite) delay before setting this
>    bit.
>  
> +  FEATURES_OK (8) Indicates that the driver has acknowledged all the
> +  features it understands, and feature negotiation is complete.
> +
>    DRIVER_OK (4) Indicates that the driver is set up and ready to
>    drive the device.
>  
> @@ -444,23 +447,44 @@ how to communicate with the specific device.
>  
>  3. The DRIVER status bit is set: we know how to drive the device.
>  
> -4. Device-specific setup, including reading the device feature
> -  bits, discovery of virtqueues for the device, optional per-bus
> -  setup, and reading and possibly writing the device's virtio
> -  configuration space.
> -
> -5. The subset of device feature bits understood by the driver is
> -   written to the device.
> -
> -6. The DRIVER_OK status bit is set.
> -
> -7. The device can now be used (ie. buffers added to the
> -   virtqueues)[4]
> +4. Device feature bits are read, and the the subset of feature bits
> +   understood by the OS and driver is written to the device.
> +
> +5. The FEATURES_OK status bit is set.

My idea, and reason I added two points (also at DRIVER)
was because suspected drivers might want to set/clear
features dependent on device specific configuration.

E.g. enable multiqueue but only if there are enough VQs
and MSI vectors to make this useful.

Maybe that's overkill?

> +
> +6. The status byte is re-read to ensure the FEATURES_OK bit is still
> +   set: otherwise, the device does not support our subset of features
> +   and the device is unusable.
> +
> +7. Device-specific setup, including discovery of virtqueues for the
> +   device, optional per-bus setup, reading and possibly writing the
> +   device's virtio configuration space, and population of virtqueues.
> +   The device is now "live".
> +
> +8. The DRIVER_OK status bit is set.  This is simply informational, 
> +   as the device may already be active at this point.
>  
>  If any of these steps go irrecoverably wrong, the guest should
>  set the FAILED status bit to indicate that it has given up on the
>  device (it can reset the device later to restart if desired).
>  
> +Devices should support all valid combinations of features, but we know
> +that implementations may well make assuptions that they will only be
> +used by fully-optimized drivers.  The resetting of the FEATURES_OK flag
> +provides a semi-graceful failure mode for this case.
> +
> +2.2.1.1. Legacy Interface: Device Initialization
> +---------------------------
> +Legacy devices do not support the FEATURES_OK status bit, and thus did
> +not have a graceful way for the device to indicate unsupported feature
> +combinations.  It also did not provide a clear mechanism to end
> +feature negotiation, which meant that devices finalized features on
> +first-use, and no features could be introduced which radically changed
> +the initial operation of the device.
> +
> +The result was the steps 5 and 6 were omitted, and steps 4 and 7
> +were conflated.
> +
>  2.2.2. Device Operation
>  ----------------------
>  
> @@ -2863,10 +2887,6 @@ the non-PCI implementations (currently lguest and S/390).
>  
>  [3] The actual value within this range is ignored
>  
> -[4] Historically, drivers have used the device before steps 5 and 6.
> -This is only allowed if the driver does not use any features
> -which would alter this early use of the device.
> -
>  [5] ie. once you enable MSI-X on the device, the other fields move.
>  If you turn it off again, they move back!
>  
> 
> Cheers,
> Rusty.


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